In recent years, the bandwidths of modulation signals have increased in radio-frequency communications standards for high-speed transmission, which means that the characteristic of modulation accuracy tends to degrade because of increase in the frequency deviation of a radio communication circuit for one communication channel. Therefore, when a mixing circuit for converting a radio-frequency signal into, for example, an intermediate-frequency (IF) signal is used in a wide band, it is important to decrease the frequency deviation of its conversion gain.
In the following description, the ratio of the amplitude of differential IF signals that are output from a mixing circuit to that of a radio-frequency signal that is input to the mixing circuit will be referred to as a conversion gain.
For example, a mixing circuit shown in FIG. 12 (refer to Patent document 1, for example) is known as a prior art technique for decreasing the frequency deviation of a conversion gain. FIG. 12 shows the circuit configuration of a conventional mixing circuit 10. The mixing circuit 10 shown in FIG. 12 includes a single-ended matching circuit MA11, an amplification circuit AP11, a differential matching circuit MA12 which includes a balun DR11, and a mixing circuit MX11.
In the mixing circuit 10 shown in FIG. 12, a single-ended radio-frequency signal that is input to an input terminal RFIN is subjected to impedance matching in the single-ended matching circuit MA11 and then amplified by a transistor M11 of the amplification circuit AP11. The single-ended radio-frequency signal as amplified by the amplification circuit AP11 is converted into differential radio-frequency signals by the balun DR11. Of the differential signals, a positive-phase (positive) output signal is subjected to matching in an inductor L11 and a resistor R11 and a negative-phase (negative) output signal is subjected to matching in an inductor L12 and a resistor R12, whereby maximum-level differential signals are input to the mixing circuit MX11.
The differential signals that are output from the differential matching circuit MA12 are down-converted by the mixing circuit MX11 being mixed with local signals that are input via a positive input terminal Lop and a negative input terminal Lon, respectively. Of the differential IF signals generated through the down conversion, a positive-phase IF signal is output from a positive output terminal IFp and a reverse-phase IF signal is output from a negative output terminal IFn.
However, inductors and capacitors (e.g., inductors L11 and L12 and capacitor C11) used as the matching elements of the single-ended matching circuit MA11 and the differential matching circuit MA12 of the mixing circuit 10 shown in FIG. 12 have large frequency deviations, which is a factor in causing a large frequency deviation of the conversion gain of the mixing circuit 10. In view of this, in the mixing circuit 10, the resistors R11 and R12 are added so as to be series-connected to the inductors L11 and L12 of the differential matching circuit MA12, respectively. As a result, the Q value of the differential matching circuit MA12 is decreased and the frequency deviation of the conversion gain of the mixing circuit 10 is decreased.